1. Field of the Invention
The present invention relates to a temperature measuring method, an object detecting method and an object detecting device with a vibrating-type level sensor. In particular, the present invention relates to a temperature measuring method, an object detecting method and an object detecting device with a vibrating-type level sensor which detects presence/absence of a target object based on an oscillation frequency of a coil of an electromagnet placed opposite to a magnet with a slight gap therebetween, the magnet being provided to a vibrating plate within a detection pipe.
2. Description of the Background Art
FIG. 10A is a schematic block diagram of a conventional vibrating-type level sensor disclosed in Japanese Patent Laying-Open No. 11-351944. Referring to FIG. 10A, a detection pipe unit 1 has its base 11 which is a fixed end and has its leading end closed with a closing unit 12 that is a free end. Within detection pipe unit 1, a thin rectangular vibrating plate 2 is provided. Vibrating plate 2 has one end fastened to closing unit 12 of detection pipe unit 1 and has the other end provided with a permanent magnet to serve as a free end. Accordingly, detection pipe unit 1, closing unit 12 and vibrating plate 2 constitute a folded cantilever in such a manner that detection pipe unit 1 is folded back at closing unit 12 to form a folded section which is vibrating plate 2.
In addition, an electromagnet 4 is attached closely to the inner wall of detection pipe unit 1 to face vibrating plate 2 in the direction of axis thereof. Electromagnet 4 is driven by an alternating current to generate a magnetic field. Between this magnetic field and a magnetic field of permanent magnet 3, an attracting/repelling action is produced to cause oscillation of the folded cantilever having vibrating plate 2, closing unit 12 and detection pipe unit 1 with base 11 serving as the fixed end.
A distortion detecting element 5 is provided on the inner wall at base 11 of detection pipe unit 1. Distortion detecting element 5 detects the state of oscillation amplitude at base 11 of detection pipe 1 to convert the oscillation amplitude into an electric signal and supply the electric signal to an amplifier circuit 6. Amplifier circuit 6 amplifies the supplied signal and provides the amplified signal again to electromagnet 4.
FIGS. 10B and 10C show attracting and repelling forces between electromagnet 4 and permanent magnet 3 caused by a current applied to electromagnet 4. Suppose that there is a relation as shown in FIG. 10B between the polarity of the current applied to electromagnet 4 and a magnetic field generated on electromagnet 4. Then, the pole of electromagnet 4 that faces permanent magnet 3 is the north pole and thus an attracting force is generated between this north pole and the south pole of permanent magnet 3 attached to vibrating plate 2 while a repelling force is generated between the north pole of electromagnet 4 and the north pole of permanent magnet 3. Consequently, the free end of vibrating plate 2 is forced and displaced upward in FIG. 10B.
On the contrary, suppose that the current applied to electromagnet 4 has the opposite polarity. Then, as shown in FIG. 10C, the pole of electromagnet 4 that faces permanent magnet 3 is of the opposite polarity, i.e., the south pole. Accordingly, this south pole repels the south pole of the permanent magnet of vibrating plate 2 while the south pole and the north pole of the permanent magnet attract each other so that the free end of vibrating plate 2 is forced downward to cause a change in oscillation state. In this way, the polarity of the current applied to electromagnet 4 can be changed according to the natural frequency of the vibrating system of the folded cantilever to produce and sustain oscillation.
According to the example shown in FIG. 10A, the oscillation of the vibrating system is detected by detecting element 5 and then converted into an electric signal which is amplified by amplifier circuit 6 and supplied again to electromagnet 4, while a detecting circuit 7 outputs a detection signal. A piezoelectric element or acceleration pickup may be employed as oscillation detecting element 5. The piezoelectric element, however, has problems in that the piezoelectric element is brittle, the piezoelectric element attached to the detection pipe with an adhesive is susceptible to environment and temperature characteristics, and thus the reliability of the piezoelectric element itself is low.
Another method, for example, disclosed in Japanese Patent Laying-Open No. 1-232218 uses a vibrating-type level detection device. This vibrating-type level detection device includes a vibrating body having an exciting piezoelectric element and a receiving piezoelectric element. The vibrating body is excited by the exciting piezoelectric element and resultant oscillation of the vibrating body is detected by the receiving piezoelectric element. An output from the receiving piezoelectric element is input to a bandpass filter which passes an oscillation frequency fa which is a frequency when the vibrating body is not in contact with a target object while the bandpass filter does not pass an oscillation frequency fb which is a frequency when the vibrating body is in contact with the target object. The output from the bandpass filter is then supplied to a voltage comparator circuit to compare the output with a reference voltage. When the vibrating body does not contact the target object, the output from the bandpass filter is lower than the reference voltage. When the vibrating body contacts the target object, the output from the bandpass filter is higher than the reference voltage. The target object can thus be detected.
As for the example of the conventional method, however, the detection circuit has a relatively larger number of components, resulting in problems of a higher cost, a more complicated structure, and increase in assembly steps. Such a large number of components also results in deterioration of the reliability.
A chief object of the present invention is to provide a temperature measuring method, an object detecting method and an object detecting device with a vibrating-type level sensor by which the number of components can be decreased and the reliability can be improved.
The present invention is a temperature measuring method with a vibrating-type level sensor measuring a temperature by utilizing characteristics of a coil of an electromagnet placed opposite to a magnet with a slight gap therebetween, the magnet being provided to a vibrating plate within a detection pipe. An alternating current at a frequency which is swept over a predetermined range centered at a resonance frequency of the vibrating plate is applied to the coil, and the temperature is measured based on a change in phase angle of a flowing current, the change in phase angle being caused by a change in resistance value of the coil due to a change in temperature of the coil.
Another invention is an object detecting method with a vibrating-type level sensor detecting presence/absence of an object based on an oscillation frequency of a coil of an electromagnet placed opposite to a magnet with a slight gap therebetween, the magnet being provided to a vibrating plate within a detection pipe. An alternating current at a frequency which is swept over a predetermined range centered at a resonance frequency of the vibrating plate is applied to the coil at every predetermined measurement cycle, a change in phase is detected that is caused according to whether or not the object contacts the vibrating plate when the alternating current is applied, presence/absence of the object is determined based on the detected change in phase, a temperature is measured based on the detected change in phase in a former half cycle of the predetermined measurement cycle and, based on a result of measurement of the temperature, the frequency which is swept is changed in a latter half cycle of the predetermined measurement cycle.
Still another invention is an object detecting device with a vibrating-type level sensor detecting presence/absence of an object based on an oscillation frequency of a coil of an electromagnet placed opposite to a magnet with a slight gap therebetween, the magnet being provided to a vibrating plate within a detection pipe. The detecting device includes an alternating current application unit for applying, to the coil at every predetermined measurement cycle, an alternating current at a frequency which is swept over a predetermined range centered at a resonance frequency of the vibrating plate, a phase detection unit for detecting a change in phase caused according to whether or not the, object contacts the vibrating plate when the alternating current is applied by the alternating current application unit, a determination unit for determining presence/absence of the object based on the detected change in phase by the phase detection unit, a temperature measurement unit for measuring a temperature based on a detection output from the phase detection unit in a former half cycle of the predetermined measurement cycle, and a frequency change unit for changing the frequency which is swept, based on a result of measurement by the temperature measurement unit in a latter half cycle of the predetermined measurement cycle.
The phase detection unit detects fluctuation of the phase caused by a beat frequency component generated in the coil of the electromagnet when the oscillation frequency of the vibrating plate and the sweep frequency of the alternating current are combined.
The phase detection unit includes a filter for extracting the beat frequency component.
The temperature measurement unit measures the temperature based on a change in phase angle of a flowing current, the change in phase angle being caused by a change in resistance value of the coil due to a change in temperature of the coil.
As discussed above, with the vibrating-type level sensor according to the present invention, an alternating current at a frequency which is swept over a predetermined frequency range centered at a resonance frequency of the vibrating plate is applied to the coil at every predetermined measurement cycle, and whether an object is present or absent is determined based on a change in phase which is caused depending on whether the object contacts the vibrating plate or not. This method of detecting an object can be employed to provide a detection unit formed of only the driving electromagnet and the permanent magnet. Accordingly, since no piezoelectric element and no acceleration pickup for a receiving sensor is required, the number of components can be decreased and the reliability of the device can be improved.
Moreover, the temperature is measured in a former half cycle of the predetermined measurement cycle based on the detected change in phase, and the range over which the frequency is swept is optimized in a latter half cycle thereof based on the measured temperature. Accordingly, the detection device can be enhanced in response.
Further, only the driving electromagnet and the permanent magnet are used as a detection unit for the temperature measuring method. No thermistor for example is newly required and thus the structure can be simplified and the cost can considerably be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.